Many quantum electronic devices, for example lasers, light amplifiers, and modulators, operate by energy excitation and subsequent radiative emission between different electronic configurations of elemental physical systems, such as atoms, molecules, or ions.
An elemental physical system (herein sometimes referred to as an EPS) is a collection of fundamental particles, typically electrons and nucleons, which can exist together in a number of distinguishable configurations (herein sometimes referred to as "states"). The simplest elemental physical systems most often considered are atoms, molecules, or ions. However, the elemental physical system could be free electrons moving in a periodic electromagnetic field. Elemental physical systems also could be electrons and holes in quantum-well structures under various electric and magnetic forces.
A hydrogen atom is a type of EPS having two fundamental particles, an electron and a proton. The electron may exist in different states, each state being distinguished by a set of quantum numbers. In one state, the hydrogen atom's electron may have one spin orientation, while in another state, the electron may have a different spin orientation. In yet another state, the electron may be in a different orbit than in the first state.
Work must be done on or extracted from an elemental physical system for that EPS to changes its state. During such a change, the physical parameters, for example spin orientation or orbital motion of electrons in the case of an ion EPS, also change. Accordingly, there is an energy associated with each different state of an EPS, and the states can be arranged in a so-called energy level scheme of increasing energy.
The state associated with the minimum energy that the EPS is capable of having is called the ground state, while states associated with higher energy levels are referred to as excited states. It should be noted that while no two states are the same, their energy levels may be; however, it is common to refer to a state by its energy level. For example, it is sometimes said that an EPS "has" an excited energy level when referring to the capability of a particular EPS to exist in a state having some level of energy associated therewith which is greater than the energy associated with the ground state; or it may be said that a particle has moved from one energy level to another, meaning that the state of the EPS has changed.
In order to simplify the conceptual basis of this invention, the following discussion refers principally to electronic transitions between electronic configurations resulting from different electronic motions in an elemental physical system. However, it is to be understood that this discussion applies equally well to transitions involving other fundamental particles such as nucleons or muons, and that configurations could be other than electronic, resulting, for example, from nuclear or muonic motions. The configurations could also result from vibrational or rotational motions, or a combination of electronic, vibrational and rotational motions.
FIG. 1 illustrates the occupation probability for three (non-degenerate) energy levels of an elemental physical system under thermal equilibrium conditions at temperature T. The probability that an electron of an elemental physical system exists in an excited energy level (level 1 or level 2) as opposed to the ground state is proportional to the Boltzmann factor EXP (-E.sub.j /kT), where k is the Boltzmann constant and E.sub.j is the energy of excitation for a particular energy level (j) relative to the ground state energy level. An excited energy level is an energy level having an energy at least kT from the ground state. Usually E.sub.j is much greater than kT meaning that the electrons are most probably located in the ground state of the elemental physical system. Even at the highest temperatures, the occupation probability decreases as the energy spacing increases from energy level zero (the ground state) to the higher numbered energy levels (the excited states).
Several methods may be used to supply energy to the system of FIG. 1 so that the probability of finding an electron in the higher energy levels is more likely. The energy can be supplied by irradiation with either coherent (laser) or incoherent (flash lamp) light sources, which is referred to as optical pumping. Electrical methods such as current injection at semiconductor interfaces (laser diodes) and voltage discharges in gases (CO.sub.2 lasers) can also be used.
FIG. 2a shows the result of supplying such energy to the system of FIG. 1. The excitation source used to supply energy to the system, pump 30, induces transitions from the ground state to excited state energy level 2 at a rate W (transitions/second). The electrons in the upper energy level states naturally tend to decay back down to the lower energy level states at some rate called the natural decay rate. Depending on the lifetimes of the electrons in the upper energy levels and the pumping strength (W), the probability of finding electrons in the upper energy levels compared to the lower energy levels may be higher, creating what is called population inversion. Systems with a population inversion between two levels exhibit optical gain in a wavelength band corresponding to a transition from the upper to the lower level. It is well known that optical gain is a prerequisite for laser action, which is usually accomplished by placing the system in an optical cavity comprising two mirrors.
FIG. 2a depicts a situation where, as a result of pumping the system, the probability of finding electrons in energy level 2 is greater than finding the electrons in energy level 1, but the probability of finding electrons in energy level 2 is smaller than the probability of finding electrons in energy level 0. This situation can occur if the lifetime of the electrons in energy level 1 is shorter than the lifetime of the electrons in energy level 2, meaning that any electrons excited into energy level 1 quickly decay down to the ground state. Since there is population inversion between level 2 and level 1, laser action is possible between level 2 and level 1.
In FIG. 2b, the probability of finding electrons in energy level 2 is approximately equal to the probability of finding electrons in energy level 0, and the probability of finding electrons in energy level 1 is greater than the probability of finding electrons in energy level 0. This situation occurs when the lifetime of the electrons in energy level 1 is much longer than the lifetime of electrons in energy level 2. Since there is population inversion between level 1 and level 0, laser action is possible between level 1 and level 0.
The method of population inversion illustrated in FIGS. 2a and 2b is a simplified illustration. The energy diagrams in FIGS. 2a and 2b are not often found in nature. Actual energy level configurations are much more complicated containing features such as degenerate or quasi-degenerate energy levels, a greater number of energy states, and band structures along with discrete states. Furthermore, certain transitions between energy levels are forbidden depending on selection rules. In addition, the method of exciting electrons to the excited energy states can involve continuous-wave or pulsed pumping systems. The pumping can cause excitation to the excited energy states directly, or indirectly using two component, or so-called donor-acceptor, systems.
Irrespective of complexity, all prior art population inversion and laser systems, except U.S. Pat. No. 4,477,906 to Case described below, have a common element--the excitation of the system involves pumping on the ground state energy level or an energy level with an energy within kT of the ground state. Pumping on an excited energy level would not be considered practical since the probability of finding electrons in excited energy levels is extremely low.
The Case U.S. Pat. No. 4,477,906 (hereinafter sometimes referred to as the '906 patent) disclosed that under certain conditions pumping on excited energy levels could produce population inversion, and even lasing.
Referring now to FIG. 2c, a generalized energy level scheme of two EPS's (A and B) is shown. The lines numbered 0-5 are the designated energy levels, but other energy levels may be present as indicated by the dashed lines. The double line arrow indicates a possible transition induced by an external pump source at rate W; curved arrows represent natural processes by which an excitation at one level decays to an excitation at a lower energy level. The up and down arrows connected by the dashed line designate an internal process taking place between neighboring EPS's called cross-relaxation. As seen in FIG. 2c, level 0 represents the ground level, level 1 represents a "pump from" or absorber level, level 2 represents an "excite to" level, level 3 indicates a "decay to" level, level 4 indicates a "decay from" level, and level 5 indicates a "pump to" level. The '906 patent discloses a system in which levels 1, 2, and 3 are the same level (i.e., levels within a kT spacing) and in which the difference in energy between levels 4 and 3 substantially equals the difference in energy between levels 2 and 0.
For example, a simplified energy level scheme is shown in FIG. 3, which is disclosed in the '906 patent. In FIG. 3, each EPS comprises at least four energy levels, with the energy spacing between the ground level (energy level 0) and the first excited state level (energy level 1) being substantially equal to the spacing between the first excited state level and the second excited state level (energy level 2). Pumping, with pump 30, between energy levels 1 and 3 induces a transition of electrons from energy level 1 to energy level 3. This arrangement is illustrated for the elemental physical system labeled A in FIG. 3. Elemental physical system A by itself will not lead to population inversion because as discussed, the probability of finding electrons in energy level 1 is low. However, the addition of a neighboring elemental physical system of the same type as A, labeled B in FIG. 3, provides a mechanism for transfer of energy between elemental physical systems A and B to increase the population of energy level 1. As examples, EPS's A and B may be fixed in close proximity (as are ions in a crystal lattice) or be neighbors for a time corresponding to a collision (as in a gas). It should be understood that both systems A and B are pumped, although FIG. 3 only illustrates pumping on system A for clarity.
Assuming that an electron in system B is excited to energy level 3 and subsequently decays down to excited energy level 2, the energy corresponding to the spacing between energy level 2 and energy level 1 of system B is transferred, as illustrated by dashed line 10, to system A, exciting an electron in the ground level of system A from the ground state to energy level 1 (arrow 12).
After transferring its energy, the electron in system B resides in energy level 1, as illustrated by arrow 14. This process results in both systems A and B having electrons in energy level 1 which are available for pump excitation. Electrons in energy level 1 which are then pumped to energy level 3 can decay down to energy level 2 to provide energy for other electrons in neighboring elemental physical systems to be excited from the ground state to energy level 1. This method builds-up the population of electrons in energy level 1. When the population of energy level 1 is built-up, a further increase in the pump rate will create a population inversion between level 2 and level 1, level 2 and level 0, or level 1 and level 0.
The process of transferring part of the energy of an electron in an excited state from one elemental physical system to a second elemental physical system to raise an electron to an excited state in the second elemental physical system is often called cross-relaxation. Cross-relaxation can be resonant or non-resonant. In a resonant cross-relaxation process, the energy lost by transition of an electron in one elemental physical system is used entirely by transitions of electrons in one or more neighboring elemental physical systems. In a non-resonant cross-relaxation process, the energy lost by one elemental physical system is not equal to the energy gain of its neighbors. The balance of energy is provided through an ancillary process, such as the absorption or emission of photons, or phonons in solid or liquid media or collisions in gases, or other energy quanta.
The mechanism for both resonant and non-resonant transfer of energy may be further classified as either radiative or nonradiative. A radiative transfer involves the emission of a photon by one elemental physical system and its subsequent absorption in another elemental physical system. Nonradiative transfers include any other energy transfers between elemental physical systems which do not predominantly involve emission of a photon by one elemental physical system and its subsequent absorption in another elemental physical system. FIG. 3 illustrates a cross-relaxation process. Whether the cross-relaxation is resonant or non-resonant is not illustrated.
It is noted that the population build-up of energy level 1 competes with the tendency of electrons to decay back to the ground state in systems A and B. Therefore, a critical pump rate must be exceeded before the population of energy level 1 becomes self-sustaining. If the pump rate is less than critical, the probability that electrons are occupying energy level 1 decreases to zero over a period of time. Above the critical pump rate, there is a so-called "avalanche" of electrons populating energy level 1.
In summary, the '906 patent discloses a method and apparatus for population inversion of EPS's, based on photon avalanche absorption in one species of EPS and in which the cross-relaxation process involves only three energy levels. The '906 patent discloses a "pump from" energy level that is an excited level and identical to one of the cross-relaxation levels.
The phenomena of cross-pumping is similar to that of cross relaxation; however, cross-pumping occurs between two elemental physical systems of different types, such as two different element dopant ions in a crystal. Using the crystal as an example, cross-pumping occurs when an electron associated with the first dopant ion is pumped from one energy level to another energy level and then decays. The decay may pass through intermediate levels, each transition between energy levels resulting in the release of energy. When energy of the appropriate quanta is released, electrons in the second dopant ion are elevated. By proper choice of the dopants, population inversion can be achieved between two energy levels in the second dopant ion.
It should be noted that in some cases in this disclosure, terminology is used which refers to creating populations of fundamental particles in one particular energy level or another. Such terminology should not be read to say that one particular energy level associated with one particular state of one particular elemental physical system (such as one ion) holds all of the fundamental particles of the population. While such may be the case in some embodiments of the invention (for example, if one considers a crystal structure to be an elemental physical system), the practice of referring to creating populations in an energy level refers also to changing the state of numerous elemental physical systems to a state associated with a particular energy level, thus actually creating a population of elemental physical systems which exist in a state corresponding to that energy level. Therefore, it should be understood that reference to populations in particular energy levels includes reference to populations of elemental physical systems which exist in a state corresponding to a given energy level.
The prior art does not teach the building of populations of fundamental particles in excited energy levels and creating population inversions that will lead to the quantum electronic systems of the present invention. As one example, it is believed that new commercially desirable up-conversion lasers will result. Up-conversion lasers generate output light at a shorter wavelength than the pump light wavelength.
Shorter wavelength lasers have improved spatial resolution. Improved spatial resolution lasers would have practical uses in laser surgery and other medical applications. Such lasers would also be useful in certain military operations because short wavelength lasers have good transmission characteristics in sea water. Furthermore, short wavelength lasers would be beneficial in optical lithography.
In addition to lasers, other optical systems may emerge which will be based on these new methods for population inversion. Optical amplifiers, optical switches, and optical logic elements are specific examples.